new imac or mac pro for soccer videography?

I really can't make up my mind here. I film soccer games for a season, consisting of 10 to 11 games. My dual 2ghz G5 isn't cutting it anymore. But I'm torn between the 3.06ghz 24" Imac or getting the quad core 2.8 mac pro. My only concern with the imac is 4 gig of memory going to be enough to run motion or color or have multiple applications (FCP and moition) open at once? Also storage, I know I can hook up external drives to it, but can FCS 2 be run off of an external drive? I just don't know how big of a difference having a desktop over the imac is going to be, if the imac can handle it. Does anyone have any advice or if you do have the new Imac's and doing video work, how is it working out for you?